From basic thermodynamic theory it is known that the efficiency of heat engines becomes higher when the temperature of the heat source increases. A constant effort toward higher temperatures in power generation has been made throughout the history of heat engines. Today, most of the power generated in the world is secured in a steam Rankine cycle where the maximum steam temperature generally does not exceed about 560.degree. C. When compared to gas turbines in which the temperature can reach about 1200.degree. C. this seems to be quite low. This limitation is necessary because where where low cost and long life of the plant are primary factors, temperature and mechanical stresses must be kept at relatively low levels. The critical components in such systems are the turbine blades and the superheater piping which both must operate at relatively high temperature and under high stress.
Another problem that limits the use of higher temperatures in heat engines is ash deposition. This is most critical in coal burning plants, the most common present power source. The temperature in the superheater of such plants, which determines the maximum steam temperature, must be low enough that the ash entrained by the burned gas will stay solid and not stick to the tubes, blocking the gas passages. At higher temperatures, generally above about 870.degree. C., the molten ash has a low enough viscosity that it flows down and does not accumulate on the tubes. This phenomenon is used in slag tap type furnaces and cyclone furnaces to remove a part of the ash from the flue gases. Parts of the furnace walls, which are water cooled, are hot enough that the ash deposited on them flows down to the slag tank. These types of boilers discharge less ash to the plant environment and need smaller ash separation equipment. If the temperature of the superheater convection bank were high enough, the flue gases would likely be even cleaner. No or very little particle emission control equipment would likely be needed.
Paradoxically, the combustion temperature in a typical furnace is very high, in some cases over 1700.degree. C. A large and costly radiant space is generally needed to actually cool the burned gases. In the cyclone furnace, where the combustion process is completed in the small space of the cyclone chamber before entering into the main large furnace, cooling the gas is the only role of the bulky and expensive radiant space.
It is highly desirable to operate at higher steam temperatures which not only will improve efficiency, but will also eliminate the need for large, expensive furnaces while helping solve the problem of controlling flue gas ash. Construction materials are available which are sufficiently durable for extended periods of time at the higher temperatures required if operated under low stress. Unfortunately, the high steam pressures in modern plants, which can reach 3500 psia with concomitant equipment stress, limit the chances that the boiler temperatures can be raised considerably using existing technology.
Combined cycles can be used in known methods for constructing high temperature boilers at relatively low pressures. See, in this regard, Thermodynamics, by Reynolds, incorporated herein by reference. In these heat engines, which have been operated successfully in laboratories, a fluid, mercury in many cases, is evaporated at very high temperatures. The vapor expands in a turbine and then condenses while transferring its heat to water in the lower Rankine cycle. The heat exchanger between the two cycles requires only a relatively small area because the heat is transferred largely by the mechanism of evaporation and condensation. While these systems use a high temperature, low pressure boiler which can be built at reasonable cost even for large power plants, they still demand very high temperature turbines which are not now available for large scale use.